The incorporation of electronic fingerprint sensors into smart phones, handheld computers, and other similar devices for securing and controlling access to the device has become prevalent. Electronic fingerprint sensors, such as capacitive fingerprint sensors, function by detecting minute changes in an electrical property within different portions of the sensor on a spatial scale corresponding to the grooves and ridges of a fingerprint pattern.
It is desirable for aesthetic and functional reasons to integrate fingerprint sensors into the display glass of mobile smart phones or other similar devices. However, actual integration of the sensor electronics, e.g., linear conductors and the like, into the display glass involves challenges both technically and in terms of manufacturing infrastructure. To do so with a substrate-based sensor (i.e., a capacitive sensor with drive conductors and sense or pickup conductors separated from the drive conductors by a dielectric substrate), for example, would require through glass vias to interconnect from top to bottom of the glass, and redistribution circuit layers on both sides of the display glass to integrate the sensor technology.
Likewise, putting sensors, such as on-chip or off-chip capacitive sensors, behind the display glass would cause a significant and typically unacceptable degradation of the signal to noise ratio (SNR) for the sensor, and likewise causes increased blurring of the imaged (sensed) fingerprint—particularly because display glass must be thick enough to maintain its mechanical integrity through normal handling of the device. Typical strengthened glasses used in smartphones have relatively low dielectric constant (D k) values, e.g., in the range of 3.5 to 10, and signal-to-noise ratio (“SNR”) degradation for a given thickness of material over a capacitive sensor is greater as the dielectric constant of the material is reduced. Thus, development of high SNR in “behind-glass” fingerprint sensors for smartphones requires the use of ultra-thin glass cover material with as small a thickness as possible or a localized region of thinned glass in the area where the sensor is located.
Unfortunately, the use of ultra-thin glass covers on mobile devices to maintain allow low thickness over finger print sensors results in a reduction in mechanical robustness of the glass upon impact, as well as undesirable increases in flexing of the glass under loads, which can cause pre-existing defects in the glass to propagate as cracks. These phenomena adversely affect reliability of the fingerprint sensor.
Existing solutions to this problem are to use a thicker glass cover over the fingerprint sensor and accept the resulting reduced SNR for the sensor with associated reduction in performance, increase in false accept rate/false reject rate (FAR/FRR), and reliability.